Trichomonas vaginalis (TV) has been associated with increased HIV vaginal shedding and HIV shedding is reduced among women who are treated for TV. Vaginal shedding of HIV can increase transmission. Proper treatment of TV, therefore, could have an impact on sexual and perinatal spread of HIV. TV recurrence rates among HIV+ women are high (i.e. 20-36%) and whether this recurrence is treatment failure or reinfection is not known. The present treatment recommendations have not been tested among HIV+ women. We propose two specific aims to examine treatment failure and reinfection. Aim # 1: To determine the if the 2 gram STAT dose of metronidazole is as effective as the 7 day 500 mg BID dose for TV treatment among HIV-infected women. This will be accomplished by conducting a randomized phase IV equivalency trial among HIV-infected women (N=508) attending two HIV outpatient clinics. Aim # 2: To describe the clinical and behavioral issues that are associate reinfection. A cohort study of TV positive women from Aim 1 who did not fail treatment (N approximately 406) will be followed at one and six months to examine risk factors for reinfection. All women at the two clinics will be screened using culture and those with TV, who are not pregnant, who are 16+ years of age and who do not have bacterial vaginosis (BV) will be eligible and will have their specimen tested for drug sensitivity. Women will be randomized to the STAT dose or the 7 day dose and they will be provided with STAT medication to deliver to their sex parters. Women will be asked to return 7 days after finishing the treatment and will be retested. Those who test TV positive will be considered treatment failures. Women who test TV negative will be followed at 1 and six months to investigate factors associated with 'einfection. The hypothesis is that the 2 gram STAT dose is equivalent to the 7 day dose and that most of recurrence is due to reinfection by an untreated sex partner. Information from this grant can be used to help tailor guidelines for the treatment of TV among HIV-infected women